The throughput of cellular communications has been dramatically raised as a result of introduction of MIMO (multiple-input multiple output) transmission techniques. However, a user terminal located at or near the boarder of the cell formed by a base station cannot obtain a high throughput due to a reduced power level of the desired radio wave and inter-cellular interferences.
So-called cooperative communication of transmitting same and identical data from a plurality of base stations that form different cells to a same user terminal is known as means for solving the above identified problem. With cooperative communication, it is possible to improve the power level of a desired radio wave and reduce interferences.
For example, when two base stations (the first base station and the second base station) are engaged in a cooperative communication and the transmitted data transmitted from each of the first base station and the second base station is x, while the received signal received by a user terminal is y, an expression of y=(H1+H2)x+n holds true, where H1 and H2 are the propagation path matrices between the first base station and the user terminal and between the second base station and the user terminal respectively and n is a noise signal. For the user terminal to determine the transmitted data x transmitted from the base stations from the received signal y, it needs to determine (H1+H2). As (H1+H2) is determined, the user terminal can realize a cooperative communication for acquiring the transmitted data.
The user terminal determines the propagation path matrix H by receiving and processing the training signal transmitted from a base station.
A training signal is defined by a specification so as to be a signal specific to the identifier for identifying a base station depending on communications standards. According to such a communication standard, the first base station and the second base station transmit respective training signals that are different from each other.
There are some user terminals that are not equipped with a feature of simultaneously processing a plurality of training signals that are different from each other when it receives such training signals. If such a terminal receives training signals that are different from each other respectively from the first base station and the second base station, it can only process the training signal transmitted from one of the base stations. Then, as a result, the terminal can determine only either of the propagation path matrices, H1 or H2. Therefore, the terminal cannot determine (H1+H2) and hence cannot realize any cooperative communication.
In an aspect, an object of the present embodiment is to provide a wire less communication device that can realize a cooperative communication when each base station has a training signal specific to it and each terminal is not equipped with a feature of simultaneously processing a plurality of different training signals.
In an aspect of the present embodiment, there is provided a control station to be connected to a first extended base station that forms a first cell and transmits data and training signals and a second extended base station that forms a second cell and transmits data and training signals, the control station including:
a first signal processor that generates a first training signal specific to the first extended base station; and
a second signal processor that generates a second training signal specific to the second extended base station;
the first signal processor and the second signal processor generating first training signals at the time of transmitting same data to a first user terminal located in an overlapping area of the first cell and the second cell byway of the first extended base station and the second extended base station for a cooperative communication;
both the training signal transmitted to the first user terminal by way of the first extended base station and the training signal transmitted to the first user terminal by way of the second extended base station at the time of cooperative communication being the first training signals.